Hands-free loudspeaker radiotelephones have been developed for use in automobiles so that the driver can talk on the radiotelephone while keeping hands free for driving. Accordingly, the driver can operate the automobile with greater concentration while talking on the radiotelephone. The driver does not need to hold a handset to the head because a loudspeaker broadcasts speech transmitted from a remote communications device being used by a remote party throughout the passenger compartment. A microphone is used to pick up the speech in the passenger compartment for transmission to the remote communications device.
An acoustic feedback path from the loudspeaker to the microphone, however, may cause undesired echoes to be transmitted to the remote communications device. Accordingly, echo suppressors have been developed for loudspeaker radiotelephones. In particular, an echo suppressor monitors an input audio signal to be applied to the loudspeaker and models an echo path of this signal from the speaker to the microphone to generate an estimate of the undesired echo portion of the output audio signal generated by the microphone. The echo estimate is then combined with the output audio signal to suppress (or reduce) the echo portion of the signal transmitted to the remote party. Echo suppression is discussed, for example, in U.S. Pat. No. 5,600,718 to Dent et al. entitled Apparatus and Method for Adaptively Precompensating for Loudspeaker Distortions, and in U.S. Pat. No. 5,680,450 to Dent et al. entitled Apparatus and Method for Canceling Acoustic Echoes Including Non-Linear Distortions in Loudspeaker Telephones. The disclosures of these patents are hereby incorporated herein in their entirety by reference.
Non-linear distortions of the input audio signal generated down stream from the point monitored by the echo suppressor, however, may not be modeled by the echo suppressor so that these distortions may result in unwanted echo and/or distortion being transmitted to the remote party. In particular, the echo suppressor may be implemented as a portion of a digital signal processor so that the input audio signal is processed in a digital form and then converted from digital form to analog form before being applied to the loudspeaker. Moreover, a power amplifier can be used to amplify the analog signal from the D-to-A converter before applying the analog signal to the loudspeaker. Accordingly, non-linear distortions caused by the power amplifier may not be accurately modeled by the echo suppressor.
In particular, the maximum output of the power amplifier generally is limited by the level of the electrical voltage supplied thereto. The electrical power supply for a loudspeaker radiotelephone in an automobile, for example, may be an automotive battery with a specified voltage output level of 1 0.8V to 15.6V. As a loss of 0.8V can be expected from the battery to the loudspeaker radiotelephone, the radiotelephone can be expected to operate from a power supply level as low as of 10V. A conventional radiotelephone may thus be designed so that the maximum allowed input signal to the power amplifier does not cause the power amplifier to generate signals having peak-to-peak amplitudes which would be clipped when the power supply is at 10 V, for example.
Situations may occur, however, where an automobile battery voltage is lower than the specified range. This may occur, for example, when the car is not running; when the battery is weak; when the alternator is not operating properly; when the engine is started; when the battery is cold; when the headlight highbeams are on; and/or when many electrical accessories are on. Accordingly, non-linear distortions may be generated by the power amplifier, and these non-linear distortions may cause an increase in echo and/or distortion transmitted to the remote party because the echo suppressor is unable to model these distortions. Moreover, these distortions may be generated at a time when reliable communications are most desired, such as when the user is having car problems.